The proposed study will test the contribution of standardized observations of test session and classroom behavior for improving assessment of Attention Deficit Hyperactivity Disorder (ADHD). Subjects will be 540 children ages 6-11, divided into matched sets of 180 with ADHD, 180 clinically referred without ADHD, and 180 nonreferred controls, with equal numbers of males and females. Examiners will use the Test Observation Form (TOF) to rate each subject's behavior during administration of the Wechsler Intelligence Scale for Children-Third Edition and the Woodcock Johnson-Ill Tests of Cognitive Abilities. Independent observers will use the Direct Observation Form (DOF) to rate the behavior of subjects in the school setting on four separate occasions. Parents will complete a structured diagnostic interview (the NIMH DISC-IV), the ADHD Rating Scale-IV (ADHDRS-IV)-Home version, Child Behavior Checklist (CBCL), and Social Skills Rating System (SSRS). Teachers will complete the ADHDRS-IV-School version, Teacher's Report Form (TRF), SSRS, and Academic Performance Rating Scale (APRS). Aims of the research are: 1) to test the validity of test session and classroom observations for differentiating clinically referred children with ADHD versus clinically referred children without ADHD versus matched nonreferred controls; 2) to test the validity of test session and classroom observations, separately and combined, for differentiating children with ADHD Inattentive subtype from children with the Hyperactive/Impulsive and Combined subtypes; 3) to test associations of test session and classroom observations with parent and teacher ratings; 4) to test associations of test session and classroom observations with measures of cognitive ability and functional impairment; and 5) to determine whether the addition of standardized test session observations and/or classroom observations to parent and teacher ratings significantly improves prediction of functional impairment due to ADHD symptoms. The findings from this study will provide a basis for improving current research and clinical practices for diagnosing ADHD and differentiating the Inattentive subtype from the other two subtypes.